In the production of natural gas (mainly containing methane) or liquefied natural gas, it is necessary to purify said natural gas stemming from a deposit, from a certain number of contaminants, in first place those which are called “acid gases”, i.e. mainly carbon dioxide (CO2) and hydrogen sulfide (H2S) but also mercaptans (R—SH), carbonyl sulfide (COS) and carbon disulfide (CS2). Carbon dioxide and hydrogen sulfide may account for a significant portion of the gas mixture stemming from a natural gas deposit, typically from 3 to 70% (in molar concentration), the other acid gases being present in smaller amounts. Many processes presently exist for de-acidifying natural gas.
A first class of processes is that of physical absorption processes, wherein the acid gases are put into contact with an absorbing solution, the transfer of the acid gases into the absorbing solution being performed by affinity, i.e. promoted by thermodynamic equilibrium. Examples of compounds which may form such suitable absorbing solutions are polyethylene glycol dimethyl ether (the “Selexol” process from UOP), propylene carbonate (a process from Fluor Corporation), N-methyl-pyrrolidone (the “Purisol” process from Lurgi), methanol (the “rectisol” process from Lurgi) or morpholine derivatives (the “morphisorb” process from UHDE). Regeneration of the absorbing solution is carried out by successive expansions at decreasing pressures, without providing any energy.
A second class of processes is that of chemical absorption processes wherein the acid gases are put into contact with an absorbing solution, the transfer of the acid gases into the absorbing solution being carried out or accelerated by a chemical reaction. Examples of compounds which may form such suitable absorbing solutions, are potassium carbonate (the “Benfield” process from UOP) and especially an alkanolamine: notably monoethanolamine (MEA), diglycolamine (DGA), diisopropanolamine (DIPA), diethanolamine (DEA), methyldiethanolamine (MDEA), activated methyldiethanolamine and triethanolamine (TEA), as well as sterically hindered amines. Regeneration of the absorbing solution is mainly carried out in a heated regeneration column.
Mention may also be made of a class of mixed physico-chemical absorption processes of such as for example the so-called “Sulfinol” process from Shell, wherein the absorbing solution is a mixture of sulfolane, water and an amine. The physical absorption processes have the drawback of being costly, not very widespread, of low efficiency when the partial pressure of acid gases is not very high and they also have the drawback of also absorbing a portion of the hydrocarbons. Moreover, the chemical or physico-chemical absorption processes have the drawback of requiring a significant provision of energy at the stage for regenerating the absorbing solution.
Conventionally, acid gases are fed to a Claus converter where the H2S is converted to sulfur but where the associated CO2 is released into the atmosphere. Therefore, there is a real need for a method for purifying gas mixtures containing acid gases which, firstly allows the CO2 stream to be produced separately from the H2S stream, and secondly is both as efficient and more economical in energy and in solvent flow rate as compared with the existing processes.